Subwoofer Alignment Guide
It’s important to choose the right driver for the right application so that the your expectations are met with the real acoustic performance of your system. TC Sounds offers subwoofer solutions for just about every type of application. No matter what size room you need to fill, in almost every case you’ll want a system that has plenty of headroom, low distortion and linear frequency response.
The frequency response curve is a direct result of the drivers parameters, box volume, system type and active equalization involved. Once you get the system response you like, you’ll also need to consider the effects of boundary gain on lower frequencies which are usually significant. A naturally sloping bass response is not necessarily a poor alignment since it may measure flatter in a room.
There are two basic types of alignments, assisted and unassisted. Unassisted alignments means no EQ (equalization) is involved and assisted means EQ filters are involved which also have their own relative Q factor that affect the frequency response. In most cases, you’ll want to achieve an in-room linear response if at all possible, but don’t’ be afraid to use EQ to fine tune the response. In fact EQ will almost always need to be used once the subwoofer system is placed in a room or car because that boundary will greatly affect final response.
Matching an amplifier with a driver is also an important step of the process; however, it’s important to know that, except for resistive matching, amplifiers are impartial to drivers. You can use any amp for any driver provided it will deliver enough voltage to reach the drivers maximum performance without amplification distortion . Having an amplifier that is too powerful for the driver is more of a myth than an actual problem, however without consideration, it can be easy to overdrive the woofer and cause mechanical damage. Its also important to know that under-powering a subwoofer will by itself never cause damage to the driver even if the amplifier is clipping. Unless the single has a very high voltage and the subwoofer is beyond its limits, then driver failure should not occur.
From movies to music, no matter what you listen to or how you listen to it, what you really want is true linear response with ultra high dynamic sound pressure levels and minimal distortion –TC Sounds is committed to getting you there!
Sealed systems have several advantages, they are typically the smallest type of system you can build, easiest, naturally protect the driver at lower frequencies with a tame 2nd order roll-off characteristic, and they are the least temperamental when it comes it linear response; However they also have one big shortcoming: They are the least efficient system, especially for low frequencies, so they can easily have the most non-linear distortion because the driver(s) has to rely exclusively on their own linear displacement to produce SPL for the entire spectrum. Compared to un-EQ’ed sealed systems, vented systems can have upwards of 6 to 9dB more SPL at tuning (usually below 40Hz) which translates in a tow to three fold advantage. At higher frequencies, sealed systems and ventend systems with the same driver and power produce similar SPL levels and will yield nearly identical upper bass frequency and impulse responses.
Sealed systems should be used if space is a consideration or if the driver you’re using has a high Qts (above 0.5) then it probably should only be used in a sealed box to prevent unwanted ringing at resonance. Low Qts drivers don’t often benefit from being used in a sealed box and can quite literally be anemic in the low end response because of the high electromotive damping factor. Assisted EQ boosts are necessary to bring up the low end response if the driver’s Qts is too low. Higher Qts drivers tend to balance nicely in unassisted sealed box and can have great low end response without Eq in the right volume.
It is important to understand that low Q drivers are not inefficient at producing low end bass in a sealed but rather their frequency response under constant voltage will be show a significant drop for low frequencies as of a result of a higher electrical dampening and very high impedance peaks. This simply means that assistance (EQ) is necessarily to issue the correct amount of voltage for linear response.
In the follow example, observe the different impedance, response and excursion curves for the following box volumes with the same driver. The large box yields greater low end response while the smaller box naturally boosts the midbass due to higher Q. The higher air pressure inside the small box works against the electromotive dampening factor and reduces it.
Vented systems, commonly called ported systems, are a combination of a direct radiating driver similar to a sealed box and a Helmholtz resonator called the port. Ported systems have an advantage over sealed systems by unloading the driver’s duties onto the port and yielding greater displacement to create high SPL over the lower more demanding octaves.
The port tuning, is a function of the driver, the port dimensions and the box volume combined; however the driver’s affect on the tuning is very mild so the port tuning frequency is mostly dominated by the dimensions of the port and box. The actual tuning frequency is defined to be the impedance minimum between he driver’s and port’s impedance peaks. In other words, the saddle between the two is when the port’s displacement is maximum and the current draw from the amplifier is also maximum. It’s important to understand that ports are not necessarily efficient in term of power consumption, in fact they are not at all, but they do offload the duties from the active driver and allow two to three times more displacement than the driver can do by itself while the system is working in the ports operating frequency – this is the fundamental advantage of the system.
Longer ports and/or larger boxes decrease tuning, while the smaller boxes or shorter ports increase tuning. It is important that the port area is suffice for the driver. High BL drivers with large cones and lots of power behind them needs lots of port area to prevent limitations and compression distortion, while a lesser driver may not need quite as much. Multiple ports work the same as one port. It is simply a construction convenience; however, it is ideal to have the least amount of friction in a port so heavy flaring on both ends is ideal. Round ports work better than slot ports, but slot ports can be a convenient construction choice.
In a vented system, the driver does indeed roll off at 24dB per octave below the tuning frequency. Also, unlike sealed systems, the driver’s excursion exponentially increases below tuning, so unless the driver is filtered (protected), mechanical damage can quite easily occur. This filter is often called a subsonic filter and should be set at or just below the tuning frequency so that the active drivers’ excursion reaches two maximum peaks, one before and one just after tuning (shown below).
In the follow example, the tuning is kept the same as the box volume is increased. Too small of a box and the system will ring and produce a hump in the response from the active driver, too large of a box and the system will peak from the port’s high Q
Passive radiator or drone systems are very similar to ported systems. They both exhibit a steep roll off below tuning, gain lots of low end sensitivity from the resonator and require approximately the same internal air volume. The difference is, ports are literally replaced with passive radiators to function as the Helmholtz resonator. Now there is not easy conversion, but there are several key differences that could make passive systems ideal. Passive systems don’t suffer from port turbulence or port resonance because they don’t require an air mass to resonant, but rather a loaded mass and piston to displace the air. These two distortion mechanisms exhibited by ported systems can increase high order harmonic distortion in a ported. Additionally, drone systems can often fit in less space since the port area + length is not a requirement. On the down side, drone radiators have a slightly deeper roll off below tune, generally are less efficient to their ported counterparts and cost more.
Passive radiators should ideally be used in pairs with a single active driver but this criteria is not always necessary. Like ports, drone radiators need to displace a lot of air – much more than the active driver, so using two or more drones to each active is often necessary to minimize compression effects.
The TC Sounds VMP is a drone radiator, but is also has a distinct advantage: It can be returned without rebuilding the box my simply loading or unloading the mass washers. Adding mass to a passive radiator is analogous to increasing the length of a port. Adjusting tuning for maximum SPL or maximum low end extension is now possible. This is the equivalent of using port plugs, but without having to reduce the port area – a valuable advantage. Passive radiators also have an advantage of being able to tune a system very low without requiring a large box. Because the mass of the drone is heavy, it dominates the resonate frequency of the system. Using a high performance driver that works in a small box can be paired with drone radiators to achieve a much lower tuning without compromising resonator displacement (port area) than an equivalent ported box.
In the follow example, the tuning is kept the same as the box volume is increased. Too small of a box and the system will ring and produce a hump in the response from the active driver, too large of a box and the system will a peak from the port’s high Q